The chemisorption of O-2 on Al(111), a model reaction for surface oxidation and catalysis, has been a long standing puzzle due to the presence of a reaction barrier in experiment, not reproduced in theory, which is often explained by non-adiabatic effects. We demonstrate in this Letter, by using Al-13(-) as a cluster model for Al(111), that self-interaction errors in the commonly used exchange-correlation functionals are responsible for the missing barrier by overestimating the charge transfer along the reaction path. Such errors should be an important consideration in the theoretical modeling of surface and cluster oxidation processes. (C) 2010 Elsevier B. V. All rights reserved.